The ever increasing anthropogenic CO2 emission due to the consumption of ˜10 TW of fossil fuel energy, accounts for over half of the enhancement in the greenhouse effect that causes global warming. International protocols targeting reduction in greenhouse gases are prompting the development of several carbon management technologies. However, economical carbon sequestration, which avoids CO2 emissions to the atmosphere, is a definitive solution. Numerous economic analyses indicate that CO2 capture dominates the cost associated with the envisaged threefold scheme, which includes capture, transportation and sequestration. CO2 separation using conventional technologies such as adsorption, absorption, cryogenic distillation, and membrane separation impose severe energy penalties, increasing the cost of electricity production by 34-75%. The CO2 capture efficiency of these processes is favored at low temperatures and high pressures. However, under actual combustion based flue gas conditions of high temperatures and low pressures, reactive separation process based on alternating carbonation and calcination reactions (CCR) of metal oxides offer unique advantages in the reduction of the overall parasitic energy consumptions.
Among the many metal oxides, calcium oxide (CaO) has been identified as the most feasible metal oxide candidate sorbent for the CCR scheme due to its high CO2 sorption capacity, low cost and its natural abundance.18 In addition, the CCR scheme can be used to maximize hydrogen production at high temperature and pressure from synthesis gas mixtures by driving the equilibrium limited Water Gas Shift Reaction (WGSR) forward through incessant CO2 removal (eqn. 2). The reaction scheme follows:Carbonation: CaO+CO2→CaCO3  (1)Water-Gas Shift Reaction: CO+H2O+CaO→CaCO3+H2  (2)Calcination: CaCO3→CaO+CO2  (3)
Several studies indicate that typical calcium sorbents do not achieve stoichiometric conversions due to the dominance of micropores which cause pore mouth closure and pore pluggage. A high reactivity Precipitated Calcium Carbonate (PCC) based sorbent, dominated by 5-20 nm sized mesopores attains near stoichiometric conversion towards many gas-solid reactions. The 10-50 micron sized PCC-CaO fines, injected at 550-700° C. for CO2 capture (eqns. 1 and 2), are separated by high temperature particulate capture devices (HT-PCD) and thermally regenerated in a separate calciner (eqn 3) to produce a sequestration ready CO2 stream. The regenerated CaO fines are re-injected for CO2 capture in the subsequent cycle. The commercial deployment of a CaO fines-based CCR process is challenged by sorbent losses in the HT-PCD and separation of these fines from fly-ash due to similar particle size distribution (PSD).
It is a goal of the present invention to produce Here we show that chicken eggshells (ES), currently an environmental nuisance, are excellent reactive agglomerates that depict sustained high reactivity towards carbonation over multiple CCR cycles. The typical dry eggshell, an excellent bioceramic composite, comprises of two predominantly calcitic (CaCO3) layers and the innermost shell membrane layer. The organic material in the eggshell has excellent calcium binding properties and leads to a strong calcitic shell by self-organizing the calcite crystals during the natural eggshell formation process. Poultry eggs, used for a variety of products, result in massive amounts of eggshell waste that incur expensive disposal costs. The average annual per capita egg consumption in the United States is about 257 in 2001. However, annual eggshell wastes from various hatcheries and egg breaking industries amount to over 190,000 tons. Current disposal options include the most basic landfill, land applications including soil mixing and organic farming, and recycling in poultry diets. Of these, landfill is the easiest option as other alternatives involve significant processing costs. Eggshells, considered as organic wastes, require about $20-40/ton for landfill disposal in the U.S. This problem is further exacerbated in European countries where land comes at a premium. In addition, landfill taxes in the United Kingdom increase this disposal cost to about £30-50/ton. Therefore, the usage of refuse eggshells in this high temperature CO2 capture technology as reactive agglomerates is simultaneously a comprehensive solution to two global environmental concerns.
This study demonstrates the novel use of refuse chicken eggshells at high temperatures as reactive agglomerates to separate CO2 from large point sources such as fossil fuel fired power plants. Acetic acid treatment simultaneously generates marketable membranes while enhancing eggshell reactivity. In addition, intermediate hydration regenerates deactivated ES sorbent thereby significantly enhancing its usage over multiple batches of CCR cycles. Eggshells overcome the engineering challenges confronting the deployment of fines based CCR process. Naturally occurring eggshells obviate the necessity to formulate expensive agglomerates from high reactivity calcium fines thereby making the process economical.
The success of this project will facilitate the development of an inexpensive direct carbon capture process from existing combustion units as well as enhance H2 production at high temperature/pressure and purity from fossil fuels such as coal. Coal reserves, amounting to 500 billion tons, are abundant in the US and are our long-term hope for domestic energy security, provided environmentally benign processes are developed for its usage. This project addresses the capture (separation) of CO2 from both combustion based flue gas and from gasification based fuel gas. Implementation of off-site CO2 sequestration schemes such as geological, mineral, and ocean sequestration can be realized only if the high cost associated with CO2 separation from flue/fuel gas is overcome. To date, amine scrubbing is the only technique being envisaged for commercial scale operation. However, numerous economical analyses have unequivocally proven that integration of amine scrubbing increased the cost of electricity produced by 50-200% on existing coal fired boilers (Herzog et al., 1997, Simbeck, 2001, Rao and Rubin, 2002). A revolutionary approach to direct capture of CO2 from flue/fuel gas involves the usage of heterogeneous non-catalytic gas solid carbonation between CO2 and calcium oxide (CaO) to form calcium carbonate (CaCO3), thereby accomplishing the separation of CO2 from the flue gas stream (Gupta and Fan, 2002). The carbonation reaction occurs as written below:CaO+CO2→CaCO3 ΔH=−178 kJ/mol
The reacted sorbent is then isolated from the flue gas and separately calcined to yield pure CO2 gas (that can be then transported to its sequestration sites) and CaO that is recycled back for further carbonation in the next cycle. This process occurs repeatedly over multiple cycles. The calcination reaction is as follows:CaCO3→CaO+CO2 ΔH=+178 kJ/mol
The flue gas generated by coal combustion typically contains 10-15% CO2, 3-4% O2, 5-7% H2O and 500-3000 ppm SO2. Primarily four gas-solid reactions can occur when CaO is exposed to flue gas from coal combustion. CaO can undergo hydration, carbonation and sulfation reactions with H2O, CO2 and SO2, respectively. In addition, SO2 can react with the CaCO3 formed due to the carbonation reaction, thereby causing direct sulfation of the carbonate. These can be stoichiometrically represented as:Hydration: CaO+H2O→Ca(OH)2  (1)Carbonation: CaO+CO2→CaCO3  (2)CaO Sulfation: CaO+SO2+½O2→CaSO4  (3)CaCO3 Sulfation: CaCO3+SO2+½O2→CaSO4+CO2  (4)Thermodynamic calculations were performed to obtain equilibrium curves for the partial pressures of H2O (PH2O), CO2 (PCO2) and SO2 (PSO2) as a function of temperature for each of these reactions using HSC Chemistry v 5.0 (Outokumpu Research Oy, Finland) (Iyer et al., 2004).
The equilibrium curves depicting the temperature dependent equilibrium partial pressures of H2O and CO2 for the hydration and carbonation reactions are shown in FIG. 1. From these equilibrium curves, we can predict that moisture does not react with CaO beyond 350° C. in the 5-7% concentration range, typical of combustion flue gas. At 10% CO2, the equilibrium temperature for CaO—CaCO3 system is 760° C. Therefore, the temperature of the carbonator needs to be kept below 760° C. in order to effect the carbonation of CaO in a 10% CO2 stream. A temperature of 700° C. offers a reasonable rate of carbonation and calcination reactions and enabled us to carry out multiple carbonation-calcination reaction (CCR) cycles under isothermal conditions. Thermodynamic data for the equilibrium temperature versus SO2 concentration for the sulfation of CaO and direct sulfation of CaCO3 are shown in FIG. 2. The SO2 concentration for the sulfation of CaO system is depicted in terms of ppmv for a total system pressure of 1 bar at 4% O2. At 700° C., the equilibrium partial pressure of SO2 is 1.84 and 5.72 ppt (parts per trillion) for the sulfation of CaO and the direct sulfation of CaCO3. Since SO2 concentration in the inlet flue gas is in the 500-3000 ppm range, sulfation of CaO and the CaCO3 will definitely occur until virtually all SO2 is consumed, leading to the formation of thermally stable CaSO4. The cumulative buildup of CaSO4 in each cycle reduces the CO2 sorption capacity of the CaO sorbent over subsequent CCR cycles (Iyer et al., 2004). This study shows the existence of an optimized residence time and temperature that maximizes XCO2/XSO2. Experiments will be conducted to quantify this parasitic sulfation on the CO2 sorption capacity for the new sorbents under consideration in this project. A possible schematic of a CaRS-CO2 process retrofit is shown in FIG. 3. Such a process ensures that the majority of the flue gas does not necessitate any change in its temperature and pressure, while energy is expended to compress CO2, which is only 5-20 vol % of the total flue gas.
Enhanced Hydrogen Production from WGSR
Fuel gas obtained from fossil fuel gasification can be subjected to the Water Gas Shift Reaction (WGSR) by the addition of steam to enhance H2 production. The WGSR can be expressed stoichiometrically as:CO+H2←→CO2+H2 ΔH=−40.6 kJ/mol  (5)However, the WGS reaction is equilibrium limited. The equilibrium constant for the WGSR, expressed as
            K      WGSR        =                            [                      CO            2                    ]                ⁡                  [                      H            2                    ]                                      [          CO          ]                ⁡                  [                                    H              2                        ⁢            O                    ]                      ,falls with increasing temperature. Hence, thermodynamics forces this reaction to be conducted catalytically in two stages: (1) high temperature shift (250-500° C.) using iron catalysts and (2) low temperature shift (210-270° C.) using copper-based catalysts (Gerhartz, 1993; Bohlbro, 1969). Membranes can separate H2 at high temperature as it is formed, thereby aiding the forward reaction (Roark et al., 2002). While numerous research endeavors have been completed to date, membrane separation remains uneconomical and the H2 separated is at low pressure as well. We propose to remove CO2 from the reacting water-gas mixture through its carbonation with CaO, thereby driving the equilibrium limited WGS reaction forward. We can thus make a higher purity H2 stream in a CO2 sequestration ready manner. Challenges in WGSR over existing fuel gas streams include the intolerance of copper-based catalysts to small quantities of sulfur (<0.1 ppm) and the relatively high steam consumption. The steam to CO ratio at 550° C. can be as high as 50 in a single-stage operation or 7.5 for the more expensive dual-stage process to obtain 99.5% pure H2 (David, 1980). Recently, Harrison and co-workers reported a single-step sorption-enhanced process that produced 97% pure H2, by steam-methane reforming (SMR) and WGSR coupled with the carbonation of dolomite (Balasubramanian et al., 1999; Lopez Ortiz and Harrison, 2001). Thermodynamics indicates that CaO can react with CO2 until the partial pressure of CO2 falls below 100 ppm at 600° C. (FIG. 1). Hence, the continuous removal of product CO2 from the WGSR chamber will incessantly drive the equilibrium-limited WGSR forward leading to stoichiometric H2 production. Besides, this reaction occurs at higher temperature (compared to WGSR), which favors fast kinetics. The steam consumption can also be drastically cut because very low CO2 concentration can be realistically achieved. Similar to flue gas, CaO can undergo hydration in water gas, thereby decreasing the net CO2 capture capacity of CaO. For effective carbonation, we have to operate in that thermodynamic regime where hydration is not feasible but carbonation takes place. For a typical gasifier in which the moisture composition ranges from 12-20 atm (PH2O), CaO hydration is prevented above 550-575° C., as seen from FIG. 1.
FIG. 4 shows that the flow of exiting water gas from the gasifier is shifted to produce more H2 through the injection of steam. The simultaneous scavenging of CO2 down to ppm levels by injection of CaO particles and further WGSR by the high temperature catalysts maximizes H2 production.
Research in the general area of sorbent synthesis has been ongoing for the past 12 years at The Ohio State University (OSU). OSU researchers have been actively developing this CaRS-CO2 process over the last four years. We have shown that the porosity of the CaO structure plays a dominant role in the carbonation kinetics and ultimate CO2 sorption capacity. CaO obtained by the calcination of naturally occurring materials such as limestone (Linwood Carbonate, LC and Linwood hydrate, LH) and dolomite are microporous in nature and unable to react to a high degree due to pore pluggage and pore mouth closure limitations (extent of carbonation: 58 wt % in the 1st cycle falling to 20 wt % in 50 cycles). OSU has patented a novel wet precipitation technique to synthesize micron sized mesoporous Precipitated Calcium Carbonate (PCC) particles (Fan et al., 1998). PCC can be obtained by bubbling CO2 gas in a Ca(OH)2 slurry in which the surface charges on the incipient CaCO3 nuclei are neutralized by the optimal addition of negatively charged polyacrylate ions (Agnihotri et al., 1999). Such a precipitate is characterized by a zero zeta potential and a maxima in surface area and pore volume. Further, its mesopore-dominated structure has shown the highest reactivity towards carbonation, sulfation and sulfidation among all available calcium-based powders (Ghosh-Dastidar et al., 1996; Chauk et al., 2000; Gupta and Fan, 2002).
PCC-CaO also is not as susceptible to loss in reactivity as LC-CaO as can be seen from FIG. 5 which depicts that PCC-CaO is able to retain a much higher reactivity even after 100 CCR cycles compared to LC-CaO or Toshiba's lithium based sorbent (Iyer et al., 2004). Calcium oxide obtained by its calcination (PCC-CaO) has achieved 68 wt % in the 1st cycle falling to 40 wt % in 50 cycles and to 36 wt % in 100 cycles. OSU's research expertise lies in the manipulation of sorbent structure that enhances the sorbent reactivity of the solids. To our knowledge, OSU's PCC-CaO sorbent is the by far the most reactive sorbent among many others detailed in literature as is evident from FIG. 5. Further, this sorbent uses rather inexpensive and environmentally benign chemicals (lime, water and CO2 gas) as opposed to the use of lithium, zirconia, organic solvents like amines, ammonia, etc that are employed by other CO2 separation processes.
PCC-CaO has also shown high reactivity for H2 generation (via carbonation of the product CO2) in the WGSR system. Preliminary results on H2 generation in simulated fuel gas conditions indicate the effectiveness of PCC-CaO over conventional LH-CaO sorbent. PCC-CaO provides 100% CO conversion for the first 240 seconds (4 min) falling to 90% by 1000 seconds and to 85% in 1600 seconds. In comparison, LH-CaO sustains 100% conversion only in the initial few seconds, dropping to 85% in 1200 seconds (20 min).
Basis for Agglomeration
To date, OSU has been successful in synthesizing and testing micron sized calcium-based sorbents. However, upon injection in the flue/fuel gas, these micron sized sorbent particles will physically mix with fly ash particles, which are also in a similar particle size range. The use of sorbent fines over multiple cycles would be hampered, as the separation of the fines from fly ash is not feasible. Ideally, the sorbent should either be large enough so that the flue/fuel gas does not entrain it and fly ash simply passes through or the sorbent particles are substantially different from fly ash particles in size such that they can be easily separated. It is also essential that the reactor design adequately address potential ash buildup issues. For example, 100-500 micron sorbent particles can be effectively separated from fly ash in a cyclone (FIG. 6a). The cyclone can be designed to allow the escape of the finer ash particles. Alternatively, 5-20 mm-sized sorbents can be used in a moving bed granular filter to remove ash particles as well (FIG. 6b). It is therefore essential to synthesize larger agglomerates from PCC fines to ensure the viability of this reaction-based separation process in a coal fired thermal power plant.
Sorbent Agglomerate Property Requirements and Prior Work
Sorbent agglomerates ranging between 0.02-50 mm in size can be synthesized from a variety of processes, such as growth and spray agglomeration, pressure compaction and thermal sintering (Sommer, 1979; McKetta, 1995; Perry, 1984). The binding mechanisms underlying the various agglomeration processes depend predominantly on the presence or absence of solid or liquid bridges (Schubert 1979; Sommer, 1988). While many processes exist for agglomeration, we have carried out substantial amount of work on sorbent compaction and binder based agglomeration. It is essential that agglomerated calcium based sorbents retain their reactivity and strength over multiple CCR cycles. Whether these agglomerates are injected into the flue/fuel gas ducts or used in moving/packed beds, they have to endure various physical, thermal and chemical strains. Further, binders used in the agglomerate formation also have to withstand high temperatures (500-900° C.) and chemical attack by moisture and acid gases such as SO2 and CO2. In entrained mode, these agglomerates are subjected to impaction and attrition due to the high velocities of flue gas streams (˜100 ft/s). This leads to generation of sorbent fines, which will again mix with fly ash. As illustrated in FIG. 7, another physical strain common to CaO sorbent particles is the molar volume change that accompanies its carbonation. The molar volume of CaCO3 is 36.9 cm3/g vs. 16.9 cm3/g for CaO, which is equivalent to a molar volumetric expansion of 2.16 (Bhatia and Perlmutter, 1983).
A detailed compaction study carried out by our group revealed that PCC compacts experience a fast decay in their capture capacities over multiple CCR testing due to the onset of mass and heat transfer problems associated with larger particles as seen in FIG. 8 (Gupta et al., 2004). Additionally, the strength of these compacts was also found to deteriorate after undergoing three CCR cycles due to the large volume change in the compact induced due to alternating carbonation and calcination of the sorbent. It was therefore important to study the possibility of incorporating binders to provide greater strength to the sorbent matrix. A number of organic and inorganic binders commonly used in the ceramics industry and in the agglomeration of limestone were used in making agglomerates. Among the inorganic binders tested were hydraulic cements, clay type materials, liquid glasses and commercially available ceramic binders. The TGA results that quantify the multicyclic reactive performance of the various agglomerates are summarized in FIG. 9. The figure shows that these binder based pellets experience a monotonic decrease in capture capacity over multiple CCR cycles similar to that obtained for the pressure compacted pellets. The loading of binder in the pellet also affects the ultimate capture capacity. For example, 10 and 20 wt % alumina binder loading in PCC achieved capture capacities of 50% and 40% respectively. Further, these alumina-based agglomerates indicate a lower drop in wt % capacity over multiple CCR cycles compared to the use of lignosulfonate bound agglomerates. While lignosulfonate bound agglomerates retain the highest reactivity, they offer no appreciable agglomerate strength. In sharp contrast, the silicate bound agglomerates (sodium/potassium silicates, ceramic epoxy, Resbond 971 and ethyl silicate) are very strong but offer hardly any reactivity. All the other binder based pellets (bentonite, alumina, titania, lignosulfonate, PVC cement, PVA) either fell apart before the drop tests could be performed, or they crumbled into powders upon testing.
In view of the present disclosure or through practice of the present invention, other advantages may become apparent.